The radiation detectors may be powder phosphor screens or needle image plates (needle IP), direct radiography detectors (amorphous silicon, amorphous selenium, Cmos (complimentary metal oxide), phosphor detector arranged for direct radiography etc.) or the like.
A radiation image is recorded on such a detector (also called ‘plate’) by exposing it to an x-ray field. The radiation image which is temporarily stored by the detector is read out in a read out system (also called ‘digitizer’), where the exposed detector, is scanned with light of an appropriate wavelength and where the image-wise modulated light emitted by the detector upon stimulation is detected and converted into a digital image signal representative of the radiation image.
The signal to noise ratio (SNR) or normalized noise power spectrum ((N)NPS) of the image data must be analysed in order to evaluate the diagnostic capacity of the radiographic system for different uniform dose levels to be able to study its behaviour over the dynamic range.
Instead of using different detectors for each dose setting, a phantom target is used that contains a number of sub-targets each with a known absorption level for x-ray exposure.
Exposure of the detector then gives an image that contains the raw data needed for SNR or (N)NPS calculation. Every sub-target contains a region of interest (roi) with a known and constant attenuation for a known X-ray beam quality (energy spectrum) and is exposed to a uniform radiation field.
It is clear then that the output of the X-ray source must be exactly known, since it determines the radiation image that is used in the quality control testing of the SNR or the (N)NPS of the digitizer.
When a dosimeter is used to measure the output of an X-ray source, or during beam quality evaluation, the environment of the dosimeter ideally has to be free of surrounding material that would scatter the X-rays to the dosimeter and corrupt the measurement. This problem is normally solved by balancing the dosimeter on a web of thin wires, or using an assembly with a telescopic arm, in an attempt to keep scattering material as far away as possible from the dosimeter.